Buggy code:
public class Concat extends Task { private static final int BUFFER_SIZE = 8192 ; private static final FileUtils FILE_UTILS = FileUtils . getFileUtils ( ) ; private static final ResourceSelector EXISTS = new Exists ( ) ; private static final ResourceSelector NOT_EXISTS = new org . apache . tools . ant . types . resources . selectors . Not ( Concat . EXISTS ) ; private File destinationFile ; private boolean append ; private String encoding ; private String outputEncoding ; private boolean binary ; private StringBuffer textBuffer ; private Resources rc ; private Vector filterChains ; private boolean forceOverwrite = true ; private Concat . TextElement footer ; private Concat . TextElement header ; private boolean fixLastLine = false ; private String eolString ; private Writer outputWriter = null ; public Concat ( ) { } public void reset ( ) { } public void setDestfile ( File destinationFile ) { } public void setAppend ( boolean append ) { } public void setEncoding ( String encoding ) { } public void setOutputEncoding ( String outputEncoding ) { } public void setForce ( boolean force ) { } public Path createPath ( ) { } public void addFileset ( FileSet set ) { } public void addFilelist ( FileList list ) { } public void add ( ResourceCollection c ) { } public void addFilterChain ( FilterChain filterChain ) { } public void addText ( String text ) { } public void addHeader ( Concat . TextElement headerToAdd ) { } public void addFooter ( Concat . TextElement footerToAdd ) { } public void setFixLastLine ( boolean fixLastLine ) { } public void setEol ( FixCRLF . CrLf crlf ) { } public void setWriter ( Writer outputWriter ) { } public void setBinary ( boolean binary ) { } private ResourceCollection validate ( ) { } public void execute ( ) { } private void binaryCat ( ResourceCollection c ) { } private void cat ( ResourceCollection c ) { OutputStream os = null ; char [ ] buffer = new char [ Concat . BUFFER_SIZE ] ; try { PrintWriter writer = null ; if ( ( outputWriter ) != null ) { writer = new PrintWriter ( outputWriter ) ; } else { if ( ( destinationFile ) == null ) { os = new LogOutputStream ( this , Project . MSG_WARN ) ; } else { File parent = destinationFile . getParentFile ( ) ; if ( ! ( parent . exists ( ) ) ) { parent . mkdirs ( ) ; } os = new FileOutputStream ( destinationFile . getAbsolutePath ( ) , append ) ; } if ( ( outputEncoding ) == null ) { writer = new PrintWriter ( new BufferedWriter ( new OutputStreamWriter ( os ) ) ) ; } else { writer = new PrintWriter ( new BufferedWriter ( new OutputStreamWriter ( os , outputEncoding ) ) ) ; } } if ( ( header ) != null ) { if ( header . getFiltering ( ) ) { concatenate ( buffer , writer , new StringReader ( header . getValue ( ) ) ) ; } else { writer . print ( header . getValue ( ) ) ; } } if ( ( c . size ( ) ) > 0 ) { concatenate ( buffer , writer , new Concat . MultiReader ( c ) ) ; } if ( ( footer ) != null ) { if ( footer . getFiltering ( ) ) { concatenate ( buffer , writer , new StringReader ( footer . getValue ( ) ) ) ; } else { writer . print ( footer . getValue ( ) ) ; } } writer . flush ( ) ; if ( os != null ) { os . flush ( ) ; } } catch ( IOException ioex ) { throw new BuildException ( ( "Error<seq2seq4repair_space>while<seq2seq4repair_space>concatenating:<seq2seq4repair_space>" + ( ioex . getMessage ( ) ) ) , ioex ) ; } finally { <START_BUG> Concat . FILE_UTILS . close ( os ) ; <END_BUG> } } private void concatenate ( char [ ] buffer , Writer writer , Reader in ) throws IOException { } private void sanitizeText ( ) { } public static class TextElement extends ProjectComponent { private String value = "" ; private boolean trimLeading = false ; private boolean trim = false ; private boolean filtering = true ; private String encoding = null ; public void setFiltering ( boolean filtering ) { } private boolean getFiltering ( ) { } public void setEncoding ( String encoding ) { } public void setFile ( File file ) throws BuildException { } public void addText ( String value ) { } public void setTrimLeading ( boolean strip ) { } public void setTrim ( boolean trim ) { } public String getValue ( ) { } } private class MultiReader extends Reader { private Reader reader = null ; private int lastPos = 0 ; private char [ ] lastChars = new char [ eolString . length ( ) ] ; private boolean needAddSeparator = false ; private Iterator i ; private MultiReader ( ResourceCollection c ) { } private Reader getReader ( ) throws IOException { } private void nextReader ( ) throws IOException { } public int read ( ) throws IOException { } public int read ( char [ ] cbuf , int off , int len ) throws IOException { } public void close ( ) throws IOException { } private void addLastChar ( char ch ) { } private boolean isMissingEndOfLine ( ) { } } }

Fix:
FileUtils . close ( os ) ;